What is wrong with 3 phase? Better than single phase any day.

Just be careful with two speed motors and some contactors, but a single speed motor with a VFD can provide several refinements with the available programming possibilities.

Nearly all my machines are over 50 years old and all the main ones are (now) running with 3 phase motors.

Edited By not done it yet on 25/06/2022 21:56:32

..no-one like Tom Seniors..?

Wanting a single phase motor is an odd priority for choosing a machine tool. Is it because 'old ways are best'? If so, it doesn't apply to single-phase motors!

Getting an electric motor to run on single-phase AC is quite difficult, and the way it's done is always a compromise. The advantage is a motor that runs off ordinary domestic electricity, but they come with many disadvantages: unreliable due to capacitors, centrifugal switches, and delicate start windings, relatively inefficient, they vibrate, and don't like being continually stopped and restarted. In short, a poor choice for a machine tool, unless the workshop only had single-phase power.

DC and 3-phase motors both outperform single-phase types but back in the day, it was expensive to convert single-phase power into DC or 3-phase. Today's electronics can do either at reasonable cost and it's unusual to find new machine tools fitted with single-phase motors. Likewise, many faced with replacing a failed single-phased motor on a old machine, choose to replace single-phase with 3-phase powered by a VFD, which provides speed control, low vibration and other significant benefits.

Another point, of all the parts on an elderly tool likely to need replacing the motor is usually, not always, the easiest. Standardised mountings have been used for years so there's a good chance a new one will just drop in. Not so the rest of the machine: bearings and other spares might be difficult to source, while wear and tear can require significant remedial work – time and money. So I'd prioritise the machine's mechanical condition above all else. The motor is bottom of the list, except watch out for machines with special motors and complicated drive arrangements designed to provide variable speed; they can be difficult to replace, rewire, and worn mechanical parts may be unobtainable or cost more than the whole machine.

Not that many different older small milling machines available. Tom Senior seem more common than Centecs, and jig borers turn up from time to time. Industry and education seem to have preferred bigger machines, especially Bridgeports, and horizontals. Mill-drilling machines of the hobby-type are a more recent arrival, and a good thing too, because they provide a choice of size from table-top to big workshop via modest shed.

Dave

If by "old tech" you mean light industrial they would quite likely have fitted 3 phase as standard rather than use inefficient, unreliable, and vibration-prone single phase.

Just don't go for anything with plain bearings or horizontal only, or Startrite, or anything thing made by Arboga as they have built in special motors.

I must admit to little feel for the following, and perhaps someone with technical experience of induction motors could comment.

A capacitor in series with the stray inductance of the start winding (or a phase shifted winding as in this case) will be series resonant at some frequency. The capacitive/inductive part of the impedance falls to zero at resonance, leaving only the residual resistances (including the effects of motor operation (if it runs)). This is inevitable, but will not be a problem providing the resonant frequency is not near 50 Hz (or whatever the supply frequency is). Clearly replacing a start capacitor with the same value is fine as the designer will already have taken care of things, and indeed the start winding path must be net capacitive for the arrangement to work properly.

Is there any chance of a problem when converting a three phase motor for single phase use? I have indeed done this, and in the absence of the means to make the appropriate measurements, I merely checked that the currents in the two paths were not too dissimilar.

Chances are, if you want 'old school', it will have to be re-imported via China or previously Taiwan in 'New clothes'. Three phase machinery was a good cheap purchase years ago when we were dismantling the British manufacturing industry. Pity (or fortuitous) inverters weren't the norm at that time.

Regards Ian.

Don't forget, if you go for inverted three phase variable speed supply, you may need an extra cooling supply to the motor.

Edited By Circlip on 26/06/2022 12:36:05

Can't find the book which might explain this so my comments may be opinion based on faulty memory and not understanding it in the first place.

I don't think resonance is a problem; it might even be ideal. The purpose of the capacitor is to introduce a phase shift so that the winding it's connected to creates a magnetic field that attracts and repels relative to the magnetic field in another winding, and the force created turns the motor. As the power of the motor depends on the strength of the magnetic field, both windings should pass the maximum current possible. Resonance, I think, creates the maximum delay possible at the capacitor – a quarter of a cycle (90 degrees). If the winding was a pure inductance, it would be possible to fry it by applying more volts and amps than it can cope with. But it's a motor, which generates a back EMF when it rotates, which limits the current flowing in the winding. I think magic smoke would only appear if the motor stalled.

Whether it's sensible or not to convert a 3-phase motor to single-phase with a capacitor is another question. I see it as a potentially useful bodge rather than a general purpose solution. The problem is 3-phase motors have three windings designed to run 120 electrical degrees apart. A capacitor only shifts by 90 degrees, so the magnetic fields produced inside the motor by it are far from optimally timed. The phase difference is enough to turn the motor, but compared with 120 degree phased power, the start torque and power output are reduced, efficiency is reduced and the motor vibrates more. May not matter if the motor starts and only has to do relatively light work, but unacceptable when hard work and big electricity bills are on the agenda. So, potentially a cheap easy way of getting a 3-phase coolant pump to run, but definitely not a sensible way of powering the lifts in a busy tower block.

Purpose built single-phase motors are more efficient, but they aren't a good choice for heavy work either.

Dave

I'd be looking for a new sparky. It may well be far to expensive to connect a 3-phase supply but I can't see the distance being a problem.There may well be 3-phase in the street anyway, so the distance to 3-phase would be a lot shorter than to the sub-station. That's what I have, a true 3-phase supply connected to the 3-phase cable buried the other side of the street.

The use of a VFD gives you the advantages of a 3-phase motor even though only a single phase supply is available.

Andrew

As per Andrew, really. But in addition, there is likely a 3 phase supply quite close – as two thirds of of the houses, nearby, will likely be fed by one of the other two phases.

Other advantages of a VFD include soft-starting, braking, safety trips and others. No need for the usual starters/overloads as the VFD replaces that item.

I agree with the sentiment that converting the three phase main drive motor of a machine to single phase with a capacitor is a poor idea. However, for something like a coolant pump motor, it seems scarcely worth the expense of a VHD if it runs well enough. This is what I converted, and anyway it was before the availability of affordable VFDs.

I perhaps did not make it clear enough what the (mild) concern is. At resonance, the inductive and capacitive reactances are equal in magnitude but opposite in phase, so they cancel and thus there would be no phase shift of the start winding current. Thus if it were possible for resonance to happen, the motor would likely not start or would run slowly, but would draw excessive current in the start winding as well as stall current in the run winding, a recipe for rapid destruction.

I just wondered whether there is any reason to caution about experimenting with a wide range of capacitor values.